The invention relates generally to a method and apparatus for producing preferably tubular extruded sections, or parisons, of thermoplastic material, by extrusion of the thermoplastic material from an extrusion head into which the plastic material is fed through an inlet orifice and from which it is discharged through an extrusion or discharge gap, the wall thickness of the extruded section being varied during extrusion in accordance with a given program.
Extruded sections of this kind may be used as parisons which are subsequently expanded in a further working operation, and processed to form a finished article, for example a bottle or flask, cask, tank or the like. When manufacturing tubular extruded sections, the outlet or discharge orifice is in the form of an annular gap. Since this manufacture is the most frequently encountered situation of use in practice, this specification generally refers hereinafter to the production of a tubular extruded section, using an annular discharge gap. It will be appreciated however that the invention is also applicable to extruded sections which are of a different cross-sectional shape, for example in connection with extruding extruded sections of strip-like or sheet-like form, in which respect different cross-sectional shapes are also possible.
It has been found that the actual length of an extruded section at the end of the extrusion operation is normally shorter or longer than the length of the extruded section which had passed through the discharge orifice of the injection head during the extrusion operation. That length, which is referred to hereinafter as the `discharge length` can be measured directly, for example using a roller which is rotated by the movement of the extruded section. However, it is also possible for the discharge length to be calculated from the speed at which the extruded section issues from the discharge orifice, and the time required for extrusion from the extrusion head of all the material forming the finished extruded section.
Variations in respect of the actual length of the extruded section at the end of the extrusion operation, in relation to the discharge length, can arise for various reasons. Thus, many thermoplastic materials have a pronounced property due to a memory effect, which results in shrinkage of the extruded section after it leaves the extrusion head, with the consequence that the actual length of the extruded section at the end of the extrusion operation is less than the discharge length. Tests have shown that the discharge length may be up to 1.8 times, and even more, the effective final length of the finished extruded section. In this connection, the forces acting on the extruded section after it leaves the extrusion head, due to the above-mentioned memory effect, very widely. They depend more particularly on the discharge speed, pressure and temperature in the system comprising the extruder and the extrusion head, the fluctuations in cross-section which the material experiences during the extrusion operation, and properties of the material. With regard to the latter consideration, account must even be taken of the fact that, when using the same material, the forces due to the memory effect may vary from one charge batch to another.
As the weight of the extruded section which hangs down from the extrusion head during the extrusion operation counteracts the forces due to the memory effect, the length and the weight of the extruded section also play a part in regard to the difference which arises between the discharge length and the effective length. The shrinkage phenomenon which is caused by such forces occurs relatively quickly after the extruded section leaves the extrusion head so that such shrinkage is no longer very great, shortly after the extruded section passes through the discharge orifice. In this respect, it should also be noted that the effect of such forces due to a memory effect decreases with increasing length of extruded section because, as already stated above, the weight of the extruded section increases with length, and the weight of the extruded sections counteracts such memory-effect forces. At any event, the effect of the forces due to a memory effect normally varies over the length of the extruded section.
Polyolefins in particular have a pronounced memory-effect property, although such property is not limited to those synthetic materials. On the other hand, there are synthetic thermoplastic materials which, when extruded, experience an increase in length during the extrusion operation so that the effective final length of a completed extruded section is greater than the discharge length. This increase in length of the extruded section during the extrusion operation is essentially to be attributed to the fact that, after the material leaves the extrusion head, it flows downwardly within the extruded section, in particular under the effect of the weight of the material itself. In this connection, it is to be assumed that the extruded section is normally extruded in a vertically downward direction. In this case also the discharge speed, pressure and temperature in the system comprising the extruder and the extrusion head, the properties of the synthetic material to be processed, and the weight of the extruded section, are essential factors in regard to the magnitude of the increase in length of the extruded section, which may vary from one extruded section to another, as was noted hereinbefore in regard to the memory-effect property of other synthetic resin materials. Typical synthetic resin materials which undergo an increase in length while being extruded are cellulose acetate and PVC, although it will be noted that this is not a complete list.
The fact that the extruded sections may be of different lengths, when using the same volume of material and the same size of discharge orifice in the extrusion head, is referred to in German Offenlegungsschrift No. 25 44 609 which describes an apparatus for eliminating or at least substantially reducing the variations in the final length of the extruded section.
It is also known for the width of the gap forming the discharge orifice of the extrusion head to be varied in accordance with a given program, during the extrusion operation, in order thereby to influence and control the wall thickness of the extruded section. This arrangement is intended to adapt the wall thickness of the finished product, for example a container which is to be blow-moulded from the extruded section in a hollow mould, to the respective requirements involved in that operation. For example, such requirements may be that the regions of the extruded section which are expanded to the greatest degree in the hollow mould under the effect of the pressure medium, which is generally gaseous, are of greater wall thickness so that they are still of sufficient wall thickness in the final product, after the blow-moulding operation has been concluded. However, producing the desired effect, namely producing predetermined wall thicknesses in given regions of the final product, presupposes that on the one hand the distribution in respect of the wall thickness in the extruded section corresponds to the wall thickness distribution in the subsequent final article, and on the other hand an extruded section which is provided with such a varying wall thickness also occupies the proper position relative to the blow-mould. The above-mentioned fluctuations in respect of the length of the extruded section therefore make it difficult to achieve a precise correlation or association between the program in respect of the wall thickness, and the extruded section itself, as generally the width of the discharge gap is adjusted in dependent on control parameters which take no account of the difference between the discharge length of the extruded section on the one hand, and the effective final length thereof, on the other hand.
Thus, it is known for the programming means for programming the variation in wall thickness to be controlled in dependence on the moement of the free end of the extruded section. It will be seen from the introductory explanation set out hereinbefore that the reduction in length which is caused in many plastic materials by the visco-resilient properties thereof and the resulting behaviour due to a memory effect of the plastic material, or the increase in length which occurs in other plastic materials during the extrusion operation has the effect that the movement of the free end of the extruded section does not normally correspond to the discharge speed of the extrusion when passing through the discharge orifice so that from the very beginning, for example when there is a constant spacing between the switching points in the programming device, which switching points produce a change in the width of the discharge orifice or change the direction in which the width of the discharge orifice varies, the corresponding points on the extruded section are disposed at unequal spacings from each other. Such spacings can also be additionally influenced at a later time in the process, by the changes in the length of the extruded section after passing through the discharge orifice, due to the visco-elastic behaviour of the material or the increase in length of the extruded section. In other words, on the one hand, applying the wall thickness program to the extruded section in regard to the variations in wall thickness over the length of the extruded section is based on incorrect presumptions, and in addition, after the wall thickness program has been applied to the extruded section, reductions or increases in the length of the extruded section may additionally influence the distribution in respect of wall thickness over the longitudinal extent of the extruded section.
Even when the programming device which programs the variations in wall thickness is controlled in dependence on other parameters, for example in dependence on time, or, when using accumulator extrusion heads, in dependence on the movement of a piston for urging the plastic material out of the accumulator chamber, it is not possible to achieve a better result, as in such cases also distortion in respect of the distribution of wall thickness over the length of the extruded section is inevitable, due to the above-mentioned factors which influence the wall thickness of the extrusion.